<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>margin注意事项一</title>
    <style>
        .outer {
          background-color: red;
          padding: 100px;
          width: 400px;
          height: 400px;
        }
        .inner {
          background-color: green;
          margin: 100px;
          width: 100px;
          height: 100px;
        }
    </style>
</head>
<body>
<!--      注意事项1: 子元素是在父元素的内容区中呈现的 -->
      <div class="outer">
        <div class="inner"></div>
      </div>
</body>
</html>